1. Field of the Invention
The present invention relates to the field of electronic crystal oscillators, and more particularly to a frequency stability control for crystal oscillators including dielectric resonator oscillators, commonly referred to as DRO which reduces the number components, increases reliability and substantially reduces size, weight, and power consumption.
2. Brief Description of the Prior Art
In the past, it has been the usual practice to stabilize the frequency of crystal oscillators through the employment of low phase noise transistors biased at their minimum noise figure operating point and an automatic gain control loop to keep the voltage amplitude across the crystal constant. The oscillator is further stabilized by employing another loop for temperature control in order to maintain the crystal and the frequency sensitive elements at the optimum temperature of the crystal""s high turnover point. For crystal oscillators with stringent long term stability requirements, an additional means is provided taking the form of a frequency control loop which is implemented with a Rubidium or Cesium cell known as the xe2x80x9cphysics packagexe2x80x9d serving as a series resonant circuit of extremely high quality (Q) factor. However, this approach to high frequency stability design is highly labor intensive, complicated and expensive due to the difficulty of circuit adjustment and number of component parts required.
In particular, when the frequency control loop is included in the xe2x80x9cphysics packagexe2x80x9d, the loop contains circuitry to excite the lamp, and a thermostat for temperature control of the lamp. Additional circuitry is required to excite the electric field inside the lamp as well as servo-amplifiers, sweep circuits, and a low frequency error signal generator. The loop further includes a lock detector, a phase modulator and frequency multipliers. This degree of complexity reduces the reliability of the oscillator and substantially contributes further to an increase in size, weight, and power consumption. Therefore, the present invention is intended to overcome the above recited shortcomings of conventional Rubidium and Cesium standards.
Also, it has been the conventional practice to frequency stabilize microwave DRO""s by phase locking the oscillator to another signal typically derived from a crystal source. This approach to high frequency stability design is highly labor intensive, complicated, expensive and inefficient because it requires successive multiplication and filtering of a low frequency signal with higher stability at the expense of higher power consumption and lower reliability. Also, as part of the multiplication process, the enhanced phase noise of the crystal source appears at the reference frequency resulting in loop design trade-offs between noise band widths and lock-in ranges.
Therefore, a long-standing need has existed to provide a DRO phase-locking technique which extends to stability enhancement of lower frequency crystal oscillators while overcoming the above problems.
Accordingly, the above problems and difficulties are avoided by the present invention which provides a novel technique to stabilize the frequency of a crystal oscillator by passing through the crystal or reflecting from the crystal two light beams. These two beams are formed from one common laser source passing through a beam splitter. After passing through the crystal or reflected from the crystal, the beams are combined and allowed to interact. Since these beams propagate through different path lengths, the result of their interaction is a beam where energy is polarized at an angle that changes if the difference in path propagation length changes. The combined beam is split again by a splitter-polarizer lens, resulting in two beams in which the energy of one contains vertical polarization only and the energy in the other horizontal polarization only. Any shift in propagation path length difference due to crystal displacement changes, causes the magnitude of these energies to change in opposite directions. The light of these polarized beams is photo-detected and converted to electric signals, and by comparing these two signals an error signal is obtained that is used to change the excitation current that passes through the crystal. The piezoelectric effect forces the crystal displacement change back to zero thus achieving frequency stability control and phase noise reduction.
Also, the present invention provides a frequency stability for a DRO or a crystal oscillator by employing a laser beam and interferometer. In another form of the invention, a laser source generates a beam to a beam splitter which then introduces the split beam to a mirror that then introduces the reflection to a dielectric resonator as well as to a beam combiner followed by introduction to a beam splitter and then through photo detectors to a phase/frequency control loop circuit. The output of the circuit is then looped back to the dielectric resonator via an interferometer including a matching network. Thereby, the phasing frequency of the dielectric resonator is adjusted by the feedback loop.
Therefore, it is among the primary objects of the present invention to provide a frequency stability means for a DRO or crystal oscillator employing a laser beam source and an interferometer.
Another object of the present invention is to provide an improved means for gaining long-term frequency stability of crystal standards or free-running DRO""s by at least one order of magnitude.
Yet another object of the present invention is to improve the phase noise ratio of crystal standards and free-running DRO""s by employing laser techniques.
Yet another object is to provide a stabilized DRO which is lighter in weight, more reliable in performance and far more economical to produce and fabricate than can be achieved with conventional phase-locked DRO""s.
A further object resides in providing a frequency standard which is lighter in weight, more reliable in performance and more economical to fabricate than conventional standards.
Also, it is among the primary objects of the present invention to provide a novel frequency stabilizing means for a crystal oscillator, which employs a laser beam and a Michelson interferometer.
Another object of the present invention is to provide a long-term frequency stability control of a crystal oscillator by improving the standards by at least one order of magnitude.
Another object of the present invention is to improve the phase noise characteristics of crystal oscillators by at least a factor or two.
Another object of the present invention is to provide a novel laser controlled crystal oscillator that reduces the size, weight and power consumption of conventional frequency standards.
Still a further object of the present invention is to provide a novel frequency control stabilization means for crystal oscillators employing a laser control feature, that is more reliable and more economical to fabricate, calibrate and maintain than otherwise possible with conventional frequency standards.